He's enjoyed her light, has basked in it.
He's tainted it.
His lovely girl, so bright was she, in the beginning. Until she let him whisk her away into his dark life. For a while it was good. He treated her like a queen, gave into her strange taste for blood bags instead of the real thing. And then he was sick of it.
So he took matters into his own hands. He dropped nearly lifeless bodies at her feet, locked her away and refused to give her anything put the vein. She'd scream and scream for hours on end. She fought and fought, trying to keep that light shining. And then she gave in.
Oh what a glorious day it had been. They reveled in the kill together, and he compared her to his Ripper, though she was slightly less violent. She tore, to rippped, she killed, and she drained. She laughed as she did it because she'd locked the beast away for so long. Klaus found himself proud. And disappointed.
He watched as the light faded into a darkness not unlike his own. Her smiles were feral, her movements and mannerisms like a beast. She snarled at him when he told her no, rebelled when he tried to control her. And she took vervain so he wouldn't compell her.
In the end he had no choice. The girl he'd so craved was gone, lost in the beast he'd forced her to become. So he had to let her go. A part of him, the possesive, jealous part, refused to simply 'let her go' though. Oh no, she must end. She must die. And he took her life.
She'd come to him, bloody and naked and wanting, and he invited her to him. He took her, he enjoyed it. When it was done he kissed her, whispered words of affection and love to which she sneered because in the life they led together there was no such thing. Only lust and blood. It stung, but he'd never admit it, as he only smiled. And then he took a stake from under his pillow, and with a gentle 'goodnight' he plunged it into her heart.
Her death brought tears of course, and regret. But there was one fact that he knew and he accepted it.
Everything has to end.